ProtobufCLabel enum

A well-formed message must have exactly one of this field.

A well-formed message can have zero or one of this field (but not more than one).

This field can be repeated any number of times (including zero) in a well-formed message. The order of the repeated values will be preserved.

This field has no label. This is valid only in proto3 and is equivalent to OPTIONAL but no "has" quantifier will be consulted.